1. Field of the Invention
The present invention relates to an aberration corrector for use in an instrument using an electron beam or ion beam (such as a scanning electron microscope or ion microprobe). The charged-particle beam is adjusted by optics to correct chromatic and spherical aberrations.
2. Description of Related Art
In a scanning electron microscope or transmission electron microscope, an aberration corrector is incorporated in the optics in order to provide high-resolution imaging or to enhance the probe current density. One proposed example of this aberration corrector uses a combination of electrostatic quadrupole elements and magnetic quadrupole elements to correct chromatic aberration. The corrector also uses four stages of octupole elements to correct spherical aberration. The principle is introduced in detail in various literature: [1] H. Rose, Optik 33, Heft 1, 1-24 (1971); [2] J. Zach, Optik 83, No. 1, 30-40 (1989); and [3] J. Zach and M. Haider, Nucl. Instru. and Meth. In Phys. Res. A 363, 316-325 (1995).
The principle of the above-described aberration corrector is described briefly now by referring to FIG. 1, where an aberration corrector C is placed ahead of an objective lens 7. The aberration corrector C comprises four stages of electrostatic quadrupole elements 1, 2, 3, 4, two stages of magnetic quadrupole elements 5, 6, and four stages of electrostatic octupole elements 11, 12, 13, 14. The two stages of magnetic quadrupole elements 5, 6 create a magnetic potential distribution analogous to the electric potential distribution created by the second and third stages of the electrostatic quadrupole elements to produce a magnetic field superimposed on the electric field. The four stages of electrostatic octupole elements 11, 12, 13, 14 create an electric field superimposed on the electric field created by the four stages of electrostatic quadrupole elements 1-4.
In an actual instrument, four stages of dipole elements and four stages of hexapole elements are also mounted to produce fields superimposed on the fields created by the aforementioned quadrupole and octupole elements. The dipole elements act as deflecting devices for axial alignment. The hexapole elements act to correct the second-order aperture aberration. Since these dipole and hexapole elements are not closely related to the present invention, they will not be described in detail below.
In this configuration, a beam of charged particles is entered from the left side as viewed in the figure. The four stages of electrostatic quadrupole elements 1-4 and the objective lens 7 together act to form a reference orbit for the beam. As a result, the beam is focused onto a specimen surface 20. In FIG. 1, both orbit Rx of the particle beam in the X-direction and orbit Ry in the Y-direction are schematically drawn on the same plane.
The reference orbit can be regarded as a paraxial orbit, that is, an orbit assumed where there is no aberration. The quadrupole element 1 causes the Y-direction orbit Ry to pass through the center of the quadrupole element 2. The quadrupole element 2 causes the X-direction orbit Rx to pass through the center of the quadrupole element 3. Finally, the quadrupole elements 3, 4 and objective lens 7 together focus the beam onto the specimen surface. In practice, these components need to be adjusted mutually for complete focusing. At this time, the four stages of dipole elements are used for axial alignment.
Referring more particularly to FIG. 1, the charged-particle beam in the X-direction orbit Rx is diverged by the quadrupole element 1 acting like a concave lens. Then, the beam is converged by the quadrupole element 2 acting like a convex lens. The beam is thus made to pass through the center of the quadrupole element 3. Then, the beam is converged by the quadrupole element 4 and travels toward the objective lens 7. On the other hand, the charged-particle beam in the Y-direction orbit Ry is converged by the quadrupole element 1 and made to pass through the center of the quadrupole element 2. Then, the beam is converged by the quadrupole element 3. Finally, the beam is diverged by the quadrupole element 4 and moves toward the objective lens 7. In this way, the function of a single concave lens is created by combining the divergent action of the quadrupole element 1 acting on the X-direction orbit Rx and the divergent action of the quadrupole element 4 acting on the Y-direction orbit Ry.
Correction of chromatic aberration using the aberration corrector C is described. To correct chromatic aberration by the system shown in FIG. 1, the potential φq2 [V] at the electrostatic quadrupole element 2 and the magnetic excitation J2 [AT] (or magnetic potential) of the magnetic quadrupole element 5 are adjusted such that the reference orbit is not affected. The whole lens system acts to correct the X-direction chromatic aberration to zero. Similarly, the potential φq3 [V] at the electrostatic quadrupole element 3 and the magnetic excitation J3 [AT] of the magnetic quadrupole element 6 are adjusted such that the reference orbit is not affected. The whole lens system acts to correct the Y-direction chromatic aberration to zero.
Correction of spherical aberration (correction of the third-order aperture aberration) is next described. Where spherical aberration is corrected, X- and Y-direction chromatic aberrations are corrected. Then, the X-direction spherical aberration in the whole lens system is corrected to zero by the potential φ02 [V] at the electrostatic octupole element 12. The Y-direction spherical aberration is corrected to zero by the potential φ03 [V] at the electrostatic octupole element 13.
Then, the spherical aberration in the resultant direction of the X- and Y-directions is corrected to zero by the electrostatic octupole elements 11 and 14. In practice, repeated mutual adjustments are necessary. Superimposition of the potentials and magnetic excitations at the quadrupole and octupole elements has been put into practical use by varying the potential or excitation applied to each pole of a single twelve-pole element by using this twelve-pole element to synthesize dipoles, quadrupoles, hexapoles, octupoles, etc. This method has been introduced, for example, in [4] M. Haider et al., Optik 63, No. 1, 9-23 (1982).
In particular, in an electrostatic design, a final stage of power supplies An (n=1, 2, . . . , 12) capable of supplying a voltage to twelve electrodes Un (n=1, 2, . . . , 12) independently is connected as shown in FIG. 9. Where a quadrupole field is produced, output voltages from a quadrupole power supply 10 are supplied to the final-stage power supplies An to obtain a quadrupole field close to an ideal quadrupole field. If it is assumed that the output voltages from the final-stage power supplies An are proportional to the output voltages from the quadrupole power supply 10, the ratio of the output voltages from the power supply 10 assumes a value as given in the reference [4] above. Where an octupole field is created to be superimposed on this quadrupole field, output voltages from an octupole power supply 18 are added to the output voltages from the quadrupole power supply 10 and supplied to the final-stage power supplies An to obtain a field close to an ideal octupole field. Similarly, a field on which a multipole field produced by a 2n-pole element (n=1, 2, . . . , 6) is superimposed is obtained using the single twelve-pole element.
In a magnetic design, a final stage of power supplies Bn (n=1, 2, . . . , 12) capable of supplying excitation currents to the coils on twelve magnets Wn (n=1, 2, . . . , 12) independently is connected as shown in FIG. 10. Where a quadrupole magnetic field is created, output voltages from a quadrupole magnetic-field power supply 15 are supplied to the final stage of power supplies Bn to produce a field close to an ideal quadrupole magnetic field. If it is assumed that the output currents from the final-stage power supplies Bn are proportional to the output voltage from the quadrupole magnetic-field power supply 15, the ratio of the output voltages from the power supply 15 assumes a magnetic exciting ratio as given in the reference [4] above. Superimposition of multipole fields other than a quadrupole magnetic field is not explained herein. However, multipole fields other than a quadrupole magnetic field can be superimposed in the same way as in the electrostatic design, by adding voltages for multipole fields to the input voltage to the final-stage power supplies Bn. A yoke for magnetically connecting the outside portions of the magnets Wn is omitted in FIG. 10.
Where electrostatic and magnetic designs are superimposed, a conductive magnetic material may be used so that the magnets Wn can act also as the electrodes Un. In this case, the coils on the magnets are mounted so as to be electrically isolated from the electrodes.
In the description given below, the 2n-pole elements are treated as if they were superimposed on top of each other to simplify the explanation. In practice, superimposition of multipole fields on a single twelve-pole field is achieved by adding voltage signals as mentioned previously.
After the end of chromatic aberration, it may be necessary to correct the second-order aperture aberration by means of four stages of hexapole elements before correction of spherical aberration is performed. This correction is made in the same procedure as in the aforementioned correction of spherical aberration. This second-order aperture aberration occurs depending on the mechanical accuracy of the aberration corrector. Normally, the amount of correction is small, and this aberration affects higher-order aberrations only a little within the scope of the present invention. The second-order aperture aberration is corrected within the aberration corrector. If the resultant magnification (described later) of the aberration corrector and the objective lens is varied, higher-order aberrations are affected little, though the resultant magnification is important in the present invention. Therefore, description of the correction of the second-order aperture aberration is omitted herein.
Potential or voltage φ used in the following description regarding electrostatic multipole elements indicates a positive value of the multipole elements arranged normally as shown in FIGS. 2(a) and 2(b). Similarly, magnetic excitation J [AT] of the magnetic type indicates magnetic excitation on the positive side.
The manner in which the angular aperture of the probe striking the specimen surface 20 is next described. After at least one of chromatic and spherical aberrations is corrected as mentioned previously, it is necessary to make appropriate the angular aperture αf of the probe striking the specimen to minimize the effects of other factors (e.g., the brightness of the electron gun located ahead of the objective aperture 8 of FIG. 1, the aberration in the condenser lens system left uncorrected because the operating conditions are modified after completion of correction, diffraction aberration, and the fourth- and higher-order aberrations). In the past, an objective aperture using an aperture plate having plural apertures of different diameters is placed before or behind an aberration corrector, and the operator selects apertures of appropriate diameters to vary the angular aperture αf in increments to seek an optimum value.
The aforementioned theory of aberration correction and experimental results are superb. However, an obstacle to commercialization is that there is no means for optimizing the angular aperture αf of the probe striking the specimen at all times if any one or more of various operating parameters, including accelerating voltage, working distance, and probe current, are modified after chromatic or spherical aberration is corrected. Where the angular aperture is adjusted, the only one available method consists of placing an objective aperture having selectable aperture diameters as mentioned previously before or behind the aberration corrector and adjusting the angular aperture in increments.
With this instrument, the angular aperture can be optimized only under certain conditions, if the aforementioned operating parameters are modified. Therefore, it has been impossible to constantly adjust the angular aperture in such a way that the probe diameter is reduced to a minimum or a desired or maximum depth of focus is achieved, in spite of the effort to correct aberrations.
Where the conventional angular aperture control lens normally inserted between the system of condenser lenses and the objective lens is placed between an aberration corrector and the objective lens intact, if the angular aperture is adjusted after aberration correction, the magnification of the lens system is varied thereby. This upsets the conditions for achieving aberration correction. Hence, this method cannot be used for the control of the angular aperture in the original sense of the phrase.
Furthermore, with the aberration corrector designed to maintain constant the excitation of magnetic quadrupole elements for correcting chromatic aberration to prevent the magnetic field from drifting if the operating parameters including the accelerating voltage are changed, the angular aperture varies when the operating parameters are changed as described above. Therefore, it is necessary to reset the angular aperture to its optimum value.